Hi,
You would look for the voltage drop of the internal transistors at your specified current, multiply that voltage times that current, then multiply by the duty cycle. Do this for all the transistors that can turn on in the package and then add the results.
For example, if one transistor turns on for 20 percent of the time then the average power is:
P=V*I*0.20
If there is another transistor in series with that one then you have to do that transistor too, and it probably has a slightly different voltage.